The field of the present invention is automotive engine management systems which use mass air flow sensors and, more particularly, mass air flow integration systems which use a mass air flow sensor.
Mass air flow sensors have often been used to measure load in automotive fuel and ignition control systems, as described in K. H. Loesing, W. Jordan, H. Gerards, M. Henning "Mass Air Flow Meter--Design and Application"; SAE-Paper 890779, 1989. These sensors have also found uses in other applications, including measuring the breath volume of patients. Some examples of typical sensor construction can be found in the following patents, which are herein incorporated by reference:
Trageser, U.S. Pat. Nos. 3,374,673 and 3,433,069. PA0 Rodder, U.S. Pat. No. 3,735,752. PA0 Agar et al, U.S. Pat. No. 4,083,244. PA0 Lauterbach, U.S. Pat. Nos. 4,317,365 and 4,403,506. PA0 William, U.S. Pat. No. 4,363,238. PA0 Shih et al, U.S. Pat. No. 4,433,576. PA0 Stoltman et al, U.S. Pat. No. 4,445,369. PA0 Ohyama et al, U.S. Pat. No. 4,448,070. PA0 Oosuga et al, U.S. Pat. No. 4,494,405.
Sumal, U.S. Pat. No. 4,587,844.
Some of these (e.g., William, U.S. Pat. No. 4,363,238 and Agar et al, U.S. Pat. No. 4,083,244) have included directionally sensitive flow rate sensors so as to distinguish between air flow directions. However, none appear to have utilized such sensed direction to control the sense of integration, thus providing a truly accurate net flow volume or mass in an environment where rapid flow variations are experienced (e.g., an engine intake tract).
In general, in the environment of engine intake tracts, mass air flow sensors have heretofor been capable of detecting variations in air flow due to production variations and/or aging of the engine. The sensors are usually calibrated for the fastest possible response so that they can adequately measure transients in the intake tract of the engine being operated. In actual practice, however, a fast response time can be a problem for analog-to-digital converters which read the voltage from the sensors for use as an input to the engine electronic control unit.
The output of the sensor is typically sampled at particular points in the engine cycle so that every cylinder has a recent flow reading. This reading is often used as if it were a direct representation of the mass of air inducted during a particular cylinder event. (A particular cylinder event can be defined, for example, as the period between ignition frequency pulses which, of course, would vary with the number of cylinders.)
However, the air flow into the engine is not steady. It has a waveform which is related to valve events, piston speed, the acoustics of the intake manifold and, if applicable, variations in valve timing with engine r.p.m. If the waveform which represents the mass air flow rate is sampled at the same particular crank angle regardless of engine r.p.m. and engine operating conditions, the result will be that the sample will be taken at different points in the waveform as the engine speed varies. This situation becomes multi-dimensionally difficult if valve timing is capable of variation.
Several solutions have been proposed in the prior art which address this drawback. The simplest solution is to simply slow the sensor's meter response time so that a sample taken at a given point in time represents more of an average of the waveform over the cycle. This solution suffers from poor transient response. However, most systems attempt to compensate errors created by point sampling of the mass air flow sensor by use of an oxygen sensor control feedback loop to compensate for systematic errors caused by air flow sensing inaccuracies. This can be done by using cartography based on engine operating conditions to look up a compensation factor to correct the sensor reading.
A better solution, embodied in the present invention, is to use a sampling scheme which reads a fast response mass air flow sensor meter in such a way that the result of the readings represents the total net mass of air passing the meter during a specified cylinder event. In this way, fast transient response is not compromised to gain accuracy of the meter reading. This method requires that enough samples be taken at fixed crank angle intervals during a cylinder event so that the waveform is accurately represented by the resulting single summed or integrated final net value. In this way, variations in air flow caused by changing acoustics, valve events, valve timing aging or production tolerances can be automatically compensated.
In general, mass air flow sensors used to sample mass air for engine control are of the hot element anemometer type, which read mass air flow directly. Since the sensing elements in the meter of such a sensor typically are symmetrical with respect to direction of flow, mass air flow is read without regard to the direction of air flow. This makes the typical integrated flow meter signal inaccurate since only the forward flow minus the reverse flow results in the true or net flow into the engine. A hot element mass flow sensor by itself, even with an integration circuit, necessarily has the reverse flow added positively to the forward flow, thus giving a reading far larger than the actual correct net flow reading.